2 scallions thinly sliced lengthwise and place in a bowl of water with ice

Cooked rice for serving

Season fillets with kosher salt.

Combine kombu (if using), sake, soy sauce, mirin, & 3/4 C fish stock or water in a medium donabe or Dutch oven or large sauce pan. Bring to a boil, then reduce heat to low. Layer white part of bok choy, then leaves in pot. Scatter mushrooms and ginger over and place reserved fish on top. Cover pot and cook until fish is barely opaque in center, 8-10 minutes.

Spoon some rice into bowls and top with vegetables and fish, dividing evenly; ladle broth over. Drizzle with sesame oil and top with scallions.